1. Field of the Invention
This invention relates to the selective dehydration of secondary hydroxyl groups in an aliphatic diol containing equal amounts of primary and secondary hydroxyl groups when such diol is in admixture with a great excess of aliphatic monoalcohol. More particularly, this invention is directed to the selective dehydration of the secondary hydroxyl groups in said monoalcohol and diol admixture and conversion of the unsaturated product so prepared to essentially pure monoalcohol by hydrogenation. This invention is particularly directed to the removal of diol impurities from primary monoalcohols whereby to improve the characteristics of such monoalcohols.
2. Discussion of the Prior Art
It is known that alcohols, particularly medium and long chain alcohols such as those containing about 4 carbon atoms, can be synthesized by several techniques including oxaldehyde synthesis coupled with reduction of the aldehyde group to an alcohol and Ziegler synthesis using an ethylene insertion type of reaction with an aluminum alkyl, preferably aluminum triethyl, followed by oxidation of the product to alcohol. Still further, it is known to produce relatively long and intermediate chain length alcohols by Aldol condensation and reduction.
Due to the mechanisms of synthesis, the alcohols synthesized as described above, inevitably contain amounts of by-products, among these particularly olefin and carbonyl compounds. Since synthetic monoalcohols having more than about 4 carbon atoms have recently gained increased importance in industry as intermediate or starting materials for the preparation of numerous products, as, for example, wetting agents and synthetic resins, it is often desirable to eliminate these by-products of synthesis. Thus, for example, it is known to remove such impurities by hydrogenation in the presence of suitable hydrogenation catalysts.
It has been found that using monoalcohols, particularly those obtained by the Ziegler process as intermediate or starting materials, difficulties arise even though the alcohol may have undergone such a purification step. It has been found that such alcohols often have presented difficulties owing to the presence of minor quantities of dihydric alcohols, e.g. diols. In other words, the pimary monoalcohols obtained inevitably contain a small amount of diols characterised by one primary and one secondary hydroxyl group. For simplicity, such diols will be denoted "secondary diols" hereinafter. These compounds are not converted during hydrogenation and remain in the monoalcohols as impurities. They steadfastly resist the purification method and seriously affect usability of the alcohols. Sulfaction of the mixture presents an objectionable odor due to sulfated diol.
It is known that alcohols can be selectively converted whereby to dehydrate secondary hydroxyl groups of diols when such diols are in admixture with primary monoalcohols. Thus, it was proposed according to U.S. Pat. No. 3,468,965 to Wikman et al to subject the mixture of primary monoalcohol and secondary diol to the action of a copper chromite catalyst and to maintain the mixture in the presence of such catalyst while under a hydrogen pressure of at least about 7 atmospheres. While such technique is useful to some extent in decreasing the diol concentration in a monoalcohol stream, such technique is expensive owing to the nature of the catalyst and to the pressures which need be employed on the reaction mixture. Additionally, such technique does not selectively dehydrate the secondary hydroxyl group in a commercially feasible manner, because too much of the primary monoalcohol is dehydrated along with the secondary hydroxyl group, thereby consuming valuable product.
It is an object of this invention, therefore, to provide a process for the removal of secondary hydroxyl groups from secondary diols when such diols are present in an alcohol stream comprising primary monoalcohol.
It is another object of this invention, therefore, to provide a process which can be carried out at atmospheric pressure or at only slightly elevated pressures with an inexpensive and readily available catalyst.
It is still a further object of this invention, therefor, to provide a process for the conversion of secondary diols to unsaturated monoalcohols wherein during such conversion, primary monoalcohols is admixture with such secondary diols are only negligibly dehydrated.
These and other objects of this invention will become more apparent in the following disclosure.